Role of Microporosity and Nitrogen Functionality on the Surface of

4704

J. Phys. Chem. C 2008, 112, 4704-4711

Role of Microporosity and Nitrogen Functionality on the Surface of Activated Carbon in the Process of Desulfurization of Digester Gas Mykola Seredych and Teresa J. Bandosz* Department of Chemistry, The City College of New York, 160 ConVent AVe, New York, New York 10031 ReceiVed: October 24, 2007; In Final Form: December 13, 2007

Microporous coconut-shell based activated carbon was modified by heating with urea or melamine at 450 and 950 °C. The materials were characterized using adsorption of nitrogen, thermal analysis, elemental analysis, and potentiometric titration to obtain detailed information about surface chemistry and texture. Then they were used as adsorbents of hydrogen sulfide from digester gas where CO2 and CH4 are present in predominant quantities. The experiments were run in dry conditions and with preadsorbed water on the surface. The results revealed the importance of water, which, by its adsorption in small pores, eliminated the adsorption of methane, and thus caused the accessibility of adsorption centers for H2S. That water also helps in H2S dissociation. The introduction of nitrogen groups, which provide basicity needed for hydrogen sulfide dissociation, enhanced significantly the removal process. Since oxygen is not supplied to the system and only that oxygen which is chemisorbed on the surface can be used, elemental sulfur is a predominant product of surface reaction. The importance of nitrogen is in providing basicity, which is not deactivated by the presence of CO2 in the challenge gas.

1. Introduction Activated carbons with a highly developed internal surface area and porosity (especially of micro- and small mesopores) are widely used for purification of air from organic pollutants and toxins. The choice of carbon depends upon the pore size distribution of the adsorbent and the “nature” of the adsorbed molecules.1-5 In adsorption from the gas phase, mainly microporous carbon is used. Recently, much attention has been paid to nitrogen-containing activated carbons owing to their specific selectivity important for gas-adsorbent interactions.5-12 Nitrogen changes the chemical character of the carbon surface into basic and affects its degree of hydrophobicity. Even if the surface of carbon is hydrophobic, small pores enhance adsorption of water thus reducing the adsorption capacity for organic molecules. On the other hand, the changes in the surface nature imposed by nitrogen certainly increase physical adsorption of polar molecules and their specific interactions with polar species via electrostatic forces13 or via hydrogen bonding.14 Nitrogen-containing activated carbons, especially obtained by high-temperature treatment, enhance dissociation of H2S to HSions, which are oxidized to elemental sulfur. When small pores are present, oxidation proceeds further, and SO2 and H2SO4 are the main products of surface reactions.5-7,9,10,15 An increase in the amount of water (“wet” condition) or O2 on the carbon surface significantly increases adsorption/oxidation of hydrogen sulfide.7,9,10,16 However, adsorption of hydrogen sulfide from the natural gas17,18 (CO2 and CH4) is more complex compared with the adsorption from air phase.5-7,15,16 An important difference is in the fact that some active centers for H2S removal, such as alkali and alkaline earth oxides, can get deactivated by their affinity toward reactions with CO2.17 Besides, the experimental conditions (flow rate and size of the bed) also affect the conversion rate for fuel gas desulfurization. The content of gas * Corresponding author. Phone: (212)650-6017. Fax: (212)650-6107. E-mail: [email protected].

in terms of hydrocarbon specification also affects H2S removal.18,19 The steady-state conversion level is lower when other hydrocarbons are present in the challenge gas. They are adsorbed in the pore system, limiting the pore space for sulfur deposition.20 The results published in the literature demonstrated the effect of nitrogen pyridine-like forms on the adsorptive properties of activated carbons.6,9,11,14,21-24 Nitrogen within the carbon matrix can cause an increase in the surface basicity. Pyridinic-N in the carbon structure contributes one p electron to the aromatic π system and has a lone electron pair in the plane of the ring. It was shown that its presence enhances the process of hydrogen sulfide5-7,9,10 and sulfur dioxide removal.21,23 It was also demonstrated that the presence of nitrogen in the carbon structure considerably increases the carbon sorption toward anions.8,25,26 Moreover, the catalytic role of nitrogen containing species on the oxidative properties of carbon is linked to its ability to activate oxygen via formation of superoxygen ions14 and via decreasing the energy gap (ELUMO-EHOMO).13 Of course, this happens only when oxygen is present in the system. The objective of this paper is to demonstrate the complexity of digester gas desulfurization on catalytic nitrogen-containing carbonaceous adsorbents. For this purpose, relatively homogeneous, microporous carbon was chosen. Since digester gas also contains carbon dioxide and methane, the effects of competitive adsorption and acid/base interactions can be evaluated. The performance of the adsorbents is demonstrated and analyzed based on changes in their surface features, which include the surface chemistry, porosity, type and number of specific adsorption/oxidation sites, and their affinity to water. 2. Experimental Section 2.1. Materials. Commercial activated coconut shell based carbon S208C referred to as S (Calgon Carbon) was used in this study. Before modification with nitrogen-containing species, a subsample of carbon was oxidized with 50% HNO3 for 4 h

10.1021/jp710271w CCC: $40.75 © 2008 American Chemical Society Published on Web 03/06/2008

Desulfurization of Digester Gas

J. Phys. Chem. C, Vol. 112, No. 12, 2008 4705

Figure 1. H2S breakthrough curves measured in dry conditions (ED) on S carbon modified with urea (A) and melamine (B) and with 2 h prehumidification (EP) on S carbon modified with urea (C) and melamine (D).

TABLE 1: H2S Breakthrough Capacity, Amount of Water Preadsorbed and the pH Values for the Initial and Exhausted Carbons Run in Dry and Wet Conditions H2S breakthrough capacity (mg/cm3 of carbon)

Sample

(mg/g of carbon)

S S-A S-AU S-AUO S-B S-BU S-BUO S-AM S-AMO S-BM S-BMO

29.8 3.0 6.4 0.4 2.3 30.6 6.1 0.1 0.1 19.5 6.8

Dry 13.8 1.5 3.5 0.23 1.2 16.0 3.2 0.06 0.06 10.4 3.6


27.2 26.4 34.0 2.4 17.6 54.1 38.1 0.4 0.4 71.9 50.5

Wet 11.6 13.2 18.0 1.3 8.8 29.3 19.7 0.2 0.2 38.2 25.5

pH water adsorbed (mg/g) initial exhausted

25.4 165.7 205.1 58.8 133.2 224.9 203.4 49.4 92.3 253.4 206.1

10.15 9.76 9.53 8.33 9.80 10.20 9.15 8.05 8.25 9.98 9.30

9.72 9.41 9.20 8.30 9.51 9.70 8.36 8.10 8.18 9.84 8.37

10.15 9.76 9.53 8.33 9.80 10.20 9.15 8.05 8.25 9.98 9.30

9.73 8.63 8.20 8.06 9.26 3.93 5.90 7.92 8.14 8.13 5.78

TABLE 2: Carbon, Hydrogen, and Nitrogen Contents [%] in the Samples Studied sample

C

H

N

N/C

S S-AU S-AUO S-BU S-BUO S-AM S-AMO S-BM S-BMO

79.0 78.5 80.9 84.9 91.2 75.8 71.3 85.1 88.2

0.7 1.5 0.8 0.7 0.3 1.2 1.0 0.9 0.6

0.12 12.2 12.7 2.9 3.7 16.1 20.5 4.0 4.1

0.002 0.16 0.16 0.034 0.041 0.21 0.29 0.047 0.047

and then washed out with water to remove excess acid and water-soluble products of oxidation. To introduce nitrogen groups, the initial carbon and oxidized carbon (30 g) were treated with urea or melamine (20 g of urea or melamine in 100 mL of ethanol) and stirred at room temperature for 5 h. In the case of melamine treatment, the mixture was boiled to evaporate alcohol, and the carbon sample was dried at 120 °C. The samples impregnated with urea or melamine were heated in nitrogen at 10 °C/min to 450 °C or to 950 °C and maintained at these temperatures for 0.5 h. After modifications, the samples were washed with boiling water to remove any excess urea or melamine decomposition products. The carbons after treatment are referred to as S-AU, S-BU, as S-AM, S-BM, where A and B refer to heat treatment at 450 and 950 °C respectively, and U or M represent urea or melamine. The preoxidized samples are referred to with an additional letter O added to their

4706 J. Phys. Chem. C, Vol. 112, No. 12, 2008

Seredych and Bandosz

Figure 2. Dependence of the H2S removal capacity on the amount of water adsorbed during the prehumidification.

names. Thus, S-AU is S modified with urea and heated at 450 °C whereas S-BMO is S preoxidized treated with melamine and heated at 950 °C. To distinguish between the effects of thermal treatment and incorporation of nitrogen, untreated coconut shell carbon samples were heated under the same conditions as those used for impregnated samples. Those samples are referred to as S-A and S-B. 2.2. Methods. Boehm Titration. 0.5 gram of carbon sample was placed in 25 mL of the following 0.05 N solutions of sodium hydroxide and hydrochloric acid. The vials were sealed and shaken for 24 h; then, 5 mL of each filtrate was pipetted, and the excess of base or acid was titrated with HCl or NaOH. The number of all acidic sites was calculated under the assumption that NaOH neutralizes carboxyl, phenolic, and lactonic groups.27 The number of surface basic sites was calculated from the amount of hydrochloric acid that reacted with the carbon. Since nitrogen-containing groups can have pKa similar to those containing oxygen, the bases of different strength to distinguish lactonic, carboxylic, or phenolic groups were not used. pH of the Carbon Surface. Carbon powder (0.4 g) was placed in 20 mL of distilled water and equilibrated during the night. Then, the pH of the suspension was measured.

Potentiometric Titration. Potentiometric titration measurements were performed with a DMS Titrino 716 automatic titrator (Metrohm). The instrument was set at the mode when the equilibrium pH was collected. Subsamples of the materials studied of about 0.100 g in 50 mL 0.01 M NaNO3 were placed in a container thermostatted at 298 K and equilibrated overnight with the electrolyte solution. To eliminate the influence of atmospheric CO2, the suspension was continuously saturated with N2. The suspension was stirred throughout the measurements. Volumetric standard NaOH (0.1 M) was used as the titrant. The experiments were done in the pH range of 3-10. In the evaluation of the data, the SAIEUS approach leading to distributions of acidity constants was used.28,29 Thermal Analysis. TG curves were obtained using a TA instrument thermal analyzer. About 30 mg of the sample were submitted to a regular increase of temperature with a heating rate 10 °C/min while the nitrogen flow rate was 100 mL/min. Adsorption of Nitrogen. On the materials obtained, sorption of nitrogen at -196 °C was carried out using an ASAP 2010 (Micromeritics). Before the experiments, the samples were outgassed at 120 °C to constant vacuum (10-4 kPa). From the isotherms, the surface areas (BET method), total pore volumes, Vt, (from the last point of isotherm at relative pressure equal to 0.99), volumes of micropores, Vmic, mesopore volume, Vmes, along with pore size distributions were calculated. The last four quantities were calculated using density functional theory, DFT.30,31 CHN. The contents of carbon, hydrogen, and nitrogen were evaluated in the commercial Schwarzkopf lab, New York, New York. EValuation of H2S Sorption Capacity. A custom-designed dynamic test was used to evaluate the performance of adsorbents for H2S adsorption from gas streams as described in the technical literature.32 Adsorbent samples were ground (1-2 mm particle size) and packed into a glass column (length 370 mm, internal diameter 9 mm, bed volume 3 cm3). A digester gas mixture (60% CH4, 40% CO2) containing 0.1% (1000 ppm) of H2S was passed through the column of adsorbent at 0.150 L/min. The flow rate was controlled using Cole Palmer flow meters. The samples were run without or with 2 h of prehumidification with moist air (70%). As exhausted, they are referred to as ED and EP, respectively. The breakthrough of H2S was monitored using electrochemical sensors. As a breakthrough concentration, 100

TABLE 3: Peak Positions (pKa) and Numbers of Species in Millimoles per Gram (in Parentheses) for the S Series of Samples sample

pH

pK 4-5

pK 5-6

9.65

4.06 (0.125) 4.51 (0.051) 4.16 (0.051) 4.70 (0.060) 4.48 (0.045) 3.94 (0.055) 4.44 (0.140) 4.95 (0.046) 4.72 (0.036) 4.34 (0.104) 4.16 (0.129)

4.99 (0.055) 5.88 (0.050) 5.74 (0.032)

S 8.92 S-A 8.68 S-AU 8.28 S-AUO 8.82 S-B 9.01 S-BU 9.12 S-BUO 8.67 S-AM 8.23 S-AMO 9.91 S-BM 8.89 S-BMO

5.77 (0.023) 4.64 (0.109) 5.79 (0.049) 5.97 (0.057) 5.34 (0.075) 5.33 (0.067)

pK 6-7

6.22 (0.033) 6.68 (0.026)

6.86 (0.064) 6.49 (0.105)

pK 7-8 7.11 (0.209) 7.18 (0.054) 7.09 (0.146) 7.19 (0.026) 7.59 (0.016) 7.79 (0.502) 7.46 (0.110) 7.30 (0.026) 7.50 (0.253) 7.57 (0.185)

pK 8-9

8.54 (0.037) 8.56 (0.023) 9.02 (0.043)

pK 9-10

pK 10-11

9.73 (0.068) 9.71 (0.106) 9.44 (0.116) 9.71 (0.087)

all 0.485 0.298 0.345 0.229

10.30 (0.133)

0.286 0.666

8.15 (0.048) 8.28 (0.042)

9.26 (0.240) 9.73 (0.140) 9.78 (0.113) 9.89 (0.148)

0.539 0.355 0.322 0.580 0.381



Figure 4. DTG curves for the S series of samples.

TABLE 4. Results of the Boehm Titration (Numbers of Surface Groups) and Surface pH Values

Figure 3. pKa distributions for the S series of samples.

ppm was arbitrarily chosen. The adsorption capacities of each sorbent in terms of milligrams of sulfur containing gases per gram of adsorbent were calculated by integration of the area above the breakthrough curves and from the H2S concentration in the inlet gas, flow rate, breakthrough time, and mass of the sorbent. The experiments were run on as-received samples. 3. Results and Discussion The hydrogen sulfide breakthrough curves measured on our adsorbents are presented in Figure 1 for the experiments run either in dry (A, C) or wet (B, D) conditions. Although the trends observed are complex, generally speaking, water on the

sample

pH

basic [mmol/g]

acidic [mmol/g]

all [mmol/g]


10.15 9.76 9.53 8.33 9.80 10.20 9.15 8.05 8.25 9.98 9.30

0.464 0.831 1.090 0.676 0.698 1.181 0.836 0.445 0.569 1.037 0.858

0.328 0.0 0.196 0.417 0.0 0.022 0.284 0.220 0.276 0.110 0.274

0.792 0.831 1.286 1.093 0.698 1.203 1.120 0.665 0.845 1.147 1.132

surface significantly increases (doubles) the breakthrough time. Moreover, modification with melamine, especially at 950 °C, results in better adsorbents, and in the case of melamine modification, preoxidation of carbon dramatically increases the breakthrough time, while an opposite effect is noticed for urea modified counterparts. It is interesting that only heat treatment of virgin carbon, especially at 450 °C, decreases the performance of adsorbents. The effect is very dramatic when the experiments are run in dry conditions. The calculated capacities in milligrams per gram of adsorbent and in milligrams per unit volume of the carbon bed are collected in Table 1. As seen, the much worse performance than for the virgin carbon is reported on the modified samples run in dry conditions. Only one sample, treated with urea at 950 °C, shows a higher capacity than S, especially per unit volume of the bed, which is important from the point of view of real life applications. On the other hand, in wet conditions, 2-3 times higher capacities are measured than those for the


Figure 5. DTG for the samples exposed to digester gas in dry conditions (ED) and after prehumidification (EP).

virgin carbons. While all modifications with urea on the oxidized carbon but the one at 450 °C increase the performance, in the case of melamine modifications, both treatments at 450 °C result in negligible capacities. It is interesting that after that treatment at 950 °C the capacity of virgin carbon significantly diminishes. This result emphasizes even more dramatically the enhancement in the performance observed after treatment with nitrogencontaining species. The modifications applied, as intended, changed the surface chemistry. Even though the pH values listed in Table 1 are the apparent values reflecting the average number and strength of surface groups, there is an indication that the chemical nature of the surface changed. More clearly, that effect is seen if the amounts of preadsorbed water are analyzed. Compared with the initial virgin carbon, modified carbons show up to 10-fold increase in the amount of water adsorbed which is related to the increase in the number of polar groups and possible rearrangement in porosity.33-36 Once again, interesting and not expected results are found for heat-treated virgin carbons for which the amount of preadsorbed water increased. Because functional groups are expected to decompose to great extent as a result of heat treatment,37-39 that increase in water adsorption requires combined surface chemistry and porosity analysis addressed later in this paper. Since the previous studies showed beneficial effects of water and surface polarity on oxidation of hydrogen sulfide,5,15 the dependence of the capacity on the amount of water adsorbed was analyzed (Figure 2). One has to remember that, even though the equilibrium in water adsorption is likely not reached here,


Figure 6. DTG for the samples exposed to digester gas in dry conditions (ED) and after prehumidification (EP).

TABLE 5: Weight Loss from TA Analysis for the Exhausted Samples Modified with Urea and Melamine at 950 °C mass loss (∆W), % sample S-BU-ED S-BU-EP S-BUO-ED S-BUO-EP S-BM-ED S-BM-EP S-BMO-ED S-BMO-EP

200-350 °C 350-550 °C SH2S [wt %] STA [wt %] 1.01 2.09 0.75 2.29 1.46 2.75 1.14 2.27

1.95 2.71 0.17 0.93 1.07 3.93 1.98

2.88 5.09 0.57 3.59 1.84 6.77 0.64 4.75

2.96 4.80 0.92 3.22 2.53 6.68 1.14 4.25

the amount retained on the surface is a direct relation to surface polarity and water/functional group’s interactions. The data presented in Figure 2 show the linear trend for the results obtained at wet conditions with R2) 0.91. Although that trend is much less evident at dry conditions, the indirect effect of the degree of surface polarity on the retention of hydrogen sulfide can be noticed. The two points obtained for virgin carbon are excluded from the correlation owing to the expected difference in the chemical nature of the surface. The most direct effects of carbon modification are seen as an increase in the nitrogen content (Table 2). It increased up to 20% in the case of sample modified with melamine and obtained at 450 °C. For the samples obtained at 950 °C, the content of nitrogen is much less with a maximum of 4%. While at 450 °C, mainly amine type groups are expected;6,16,22 treatment at 950 °C leads to pyridinic and quaternary nitrogen.22,40 More



TABLE 6: Structural Parameters Calculated from Nitrogen Adsorption Isotherms sample

SBET [m2/g]

Vmic [cm3/g]

Vmeso [m2/g]

Vt [cm3/g]

Vmic/Vt

S S-ED S-EP S-A S-A-ED S-A-EP S-AU S-AU-ED S-AU-EP S-AUO S-AUO-EP S-AM S-AMO S-B S-B-ED S-B-EP S-BU S-BU-ED S-BU-EP S-BUO S-BUO-ED S-BUO-EP S-BM S-BM-ED S-BM-EP S-BMO S-BMO-ED S-BMO-EP

898 968 931 905 908 853 640 731 666 374 366 37 157 882 835 867 808 733 655 844 834 740 732 671 496 829 808 726

0.454 0.473 0.467 0.450 0.459 0.429 0.321 0.363 0.335 0.188 0.184 0.019 0.080 0.443 0.418 0.438 0.406 0.368 0.334 0.423 0.417 0.384 0.372 0.338 0.259 0.413 0.410 0.362

0.029 0.035 0.018 0.031 0.021 0.023 0.019 0.025 0.019 0.012 0.012 0.001 0.005 0.022 0.028 0.028 0.026 0.020 0.012 0.027 0.026 0.008 0.014 0.017 0.004 0.024 0.016 0.023

0.483 0.508 0.485 0.481 0.480 0.452 0.340 0.388 0.354 0.200 0.196 0.020 0.085 0.465 0.446 0.466 0.432 0.388 0.346 0.450 0.443 0.392 0.386 0.355 0.263 0.437 0.426 0.385

0.94 0.93 0.96 0.94 0.96 0.95 0.94 0.94 0.95 0.94 0.94 0.95 0.94 0.95 0.94 0.94 0.94 0.95 0.96 0.94 0.94 0.98 0.96 0.95 0.99 0.95 0.96 0.94

nitrogen in the preoxidized samples is the result of interactions of amines with the carboxylic groups formed on the surface.41 Changes in surface chemistry are seen in Figure 3, where the distributions of the acidity constants for the species present on the surface are shown. The peak positions and the number of groups, which they represent, are collected in Table 3. An increase in the heat temperature treatment for virgin carbon clearly results in an increase in the degree of surface heterogeneity seen as an increase in the number of peaks. Moreover, an increase in the number of species classified as strong acids, with pKa < 8,42 is accompanied by an increase in basic species (pKa > 8). Overall, compared with the virgin carbon, the number of species detected in the experimental window between 3 and 11 decreases, as expected. Treatment with nitrogen-containing species increases the surface heterogeneity, especially in the case of preoxidized samples and those modified at 450 °C. This is the result of an increased uptake of nitrogen-containing precursors owing to their interactions with the carboxylic groups formed on the surface as a result of oxidation6,41 and not complete decomposition of those bases. It is interesting that the largest number of groups exists on the surface of the carbons modified both with melamine and with urea at 950 °C, suggesting that even though the oxidation is expected to increase, the amount of bases retained on the surface, the compounds formed, decompose without an introduction of nitrogen to the carbon matrix. Another method bringing an input contributing to our view of surface chemistry of the materials studied is Boehm titration (Table 4).4 Owing to the presence of nitrogen, all acidic groups (titrated with NaOH) and basic groups (titrated with HCl) were determined. Here, the results show that heat treatment of virgin carbon destroyed its acidic groups and increased the number of basic groups. The apparent discrepancies with the potentiometric titration results are caused by detection of conjugated acids and by a narrower pH window in the latter method. Nevertheless, the trends observed are consistent in both groups of results.

Figure 7. Examples of pore size distributions for the initial and exhausted samples.

Thus, the treatment with both nitrogen containing species increases the number of basic groups to a similar extent, and after preoxidation, less basic groups and more acidic are detected when the samples are heated at 950 °C compared with the samples, which were not preoxidized. Generally, higher temperature of treatment leads to more basic groups, which can be a factor affecting sample polarity and retention of hydrogen sulfide. The changes in chemistry should affect not only the amount of H2S retained on the surface but also the chemical state of surface reactions products. Even based on the pH values for the exhausted samples listed in Table 1, the differences in the products exist, especially for preoxidized samples treated at 950 °C for which the low pH indicates the presence of sulfurous or sulfuric acid on the surface. The DTG curves for the initial samples, not exposed to digester gas are presented in Figure 4. For the sample obtained at 950 °C, the curves are almost flat, besides the peak at about 100 °C representing removal of physically adsorbed water; the sample obtained at 450 °C reveals peaks between 500 ad 800 °C, representing decomposition of nitrogen containing precursors.7,43,44 The peaks are much more intense on the preoxidized samples owing to more nitrogencontaining precursor retained on the surface and additional mass loss owing to the decomposition of oxygen functionalities. The DTG curves for the samples exposed to digester gas are presented in Figures 5 and 6. The peaks between 300 and 500 °C represent removal of elemental sulfur from pores of various sizes.5,15 The first peak represents removal of water when the sample is prehumidified. It is interesting that on the samples



Figure 8. Possible surface reactions scenario.

run in dry conditions the weight loss at the temperatures less than 200 °C has a totally different pattern than that for the prehumidified samples. This can be explained by the removal of methane and CO2 physically adsorbed in very small pores of those carbons. When water is adsorbed on the surface before exposure to digester gas, it is located in very small pores,36 and thus there are no adsorption centers, which would be able to enhance the physical adsorption of CH4 at ambient conditions. CO2 can still get dissolved to certain extent. Owing to the lack of adsorbed methane, the small pores are able to accept hydrogen sulfide, which dissociates owing to the effect of basicity provided by nitrogen groups, and then oxidation to elemental sulfur occurs. Oxygen chemisorbed on the surface must be the electron acceptor in this case since the challenge gas is oxygen free. The role of nitrogen basicity must be crucial for this process since the amount of H2S oxidized is greater in comparison to the situation when hydrogen sulfide is removed from the air phase.5-7 In the latter scenario, owing to abundance of oxygen, oxidation of sulfur to SO2 and SO3 /H2SO4 occurs which sort of “self-poisons” the surface causing unfavorable conditions for dissociation of hydrogen sulfide. This must be the limiting factor in the mechanism of this process. That elemental sulfur formed on the surface in the case of H2S removal from digester gas is seen as peaks between 200 and 500 °C. When a lot of H2S is removed, in the case of experiments run in the presence of water on the carbon surface, the well-defined two peaks are present. The first peak is shifted to lower temperature in the case of the samples, which were preoxidized. For them also, the lowest pH was measured after exhaustion (Table 1). The latter suggests that, besides sulfur, acids are formed. This is a possible scenario since extra oxygen needed for this reaction can come from surface functional entities, which do not decompose during heat treatment at 950 °C. In the case when sulfur is the oxidation product, the double feature of the decomposition pattern represents removal of sulfur from various pore sizes. Support for this is only one peak, the low temperature one, revealed when the experiments are run in dry digester gas. In this case, the smaller pores from which sulfur could be removed at higher temperature are filled by adsorbed methane, and deposition of oxidation products can occur only in larger pores, too large to retain methane. By taking into account the above discussion, the balance of sulfur was calculated based on the weight losses at two temperature ranges and on the expected amount of sulfur deposited calculated from breakthrough tests. As seen from Table 5, a very good agreement was found. The discrepancies

exist when a very small amount of H2S was adsorbed, and they can be related to the limited sensitivity of the TA method. The role of small pores in retention of hydrogen sulfide is demonstrated in Table 6 where the structural parameters calculated from nitrogen isotherms are listed. For the exhausted samples, only the results for the samples obtained at 950 °C are reported since only in their case, based on the amount of hydrogen sulfide adsorbed, the changes in porosity are expected. In all cases when the experiments were run in the presence of water, a noticeable decrease between 15 and 20% in the structural parameters, especially in the volumes of micropores, is found indicating that they are the favorite sites for surface reactions. As seen from the examples of pore size distributions (Figure 7), the volumes in pores smaller than 10 Å decreased significantly. Nevertheless, the fact that the volume of mesopores also decreases to a significant extent indicates that all surfaces are active in the oxidation process. In the case of samples modified at 450 °C, especially those with melamine, the carbon pores are clogged by deposited charred and not fully decomposed organic species.7 This apparently limits the amount of H2S adsorbed and oxidized. That high activity of the surface must be liked to the presence of nitrogen-containing species. Their introduction has a small effect on the porosity decreasing the volume of small pores in the samples obtained at high temperatures. Although that decrease can slightly enhance the adsorption potential, the increase in the capacity must be linked to the basicity provided by nitrogen species. Lack of reactivity of those groups with CO2, even though CO2 can slightly affect the pH when dissolved in small pores, is a valuable asset of these carbons’ surface chemistry. Besides providing basicity, the nitrogen containing groups can also participate in oxidation of hydrogen sulfide provided that oxygen is chemisorbed. The possible scenario is presented in Figure 8. For these reactions to occur, the dissociated form of hydrogen sulfide is needed, which may happen only in the basic environment.9 To check if the extensive factor of the number of groups has the effect on the amount of H2S oxidized, the dependence of the amount adsorbed on the number of groups was analyzed (Figure 9). The analysis was done separately for the urea and melamine modified samples run in wet conditions assuming that the nature of groups, and thus the intensity of the effect, can differ. Moreover, the intensity of the effect should also be changed with an increase in heat temperature treatment. Since the results indicate the linear trends, the basicity of all nitrogen species is apparently strong enough to enhance dissociation of


Figure 9. Dependence of capacity on the number of basic groups for carbon treated with urea (U) and melamine (M).

hydrogen sulfide. In such a case, the differences in the capacity seem to be linked to the distribution of those nitrogen species and their accessibility, which is related to their surface area and porosity. For instance, the small capacity for S-AM and S-AMO can be linked to the very small surface area and pore volume in these materials. The bigger slope in the case of melamine modification than in the case of urea suggests the stronger effect of the melamine-derived basicity. Similar conclusions were presented by Bagreev at al.7 Conclusions The results presented in this paper emphasize the role of carbon surface features and experimental conditions in the process of digester gas desulfurization. The nitrogen-derived basic species incorporated to the carbon matrix give rise to basicity needed for efficient H2S dissociation and its oxidation, provided that water is present in the system. The importance of water in this case is also in its ability to be adsorbed in small pores. When those pores are filled with water, they become inaccessible for methane and carbon dioxide molecules. Their competitive adsorption limits the number of adsorption centers for H2S when the experiments are run in dry conditions. References and Notes (1) Bansal, R. C.; Donnet, J. B.; Stoeckli, F. ActiVe Carbon; Marcel Dekker: New York, 1988. (2) Gregg, S. J.; Sing, K. S. W. Adsorption, surface area, and porosity; Academic Press: New York, 1982. (3) Leon y Leon, C. A.; Radovic, L. R. Interfacial chemistry and electrochemistry of carbon surfaces. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1992; Vol. 24, pp 213310. (4) Boehm, H. P. Carbon 1994, 32, 759-769. (5) Bandosz, T. J. Desulfurization on activated carbons. In ActiVated carbon surfaces in enVironmental remediation; Bandosz, T. J., Ed.;

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Role of Microporosity and Nitrogen Functionality on the Surface of

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